The present invention relates to any digital wireless mobile system which employs a training sequence. More specifically, the present invention relates to a demodulation technique for decoding a received signal under channel impairment conditions.
In digital wireless mobile system technology demodulating a received signal under channel impairment conditions may require the use of specialized digital signal processing techniques. Demodulation in this case refers to the extraction of a message signal from a modulated carrier after it has propagated through a transmission (radio) channel.
In wireless systems, the radio channel may impose severe signal impairments. For example, the mobility of a wireless unit causes channel variations known as Doppler effect. Moreover, the reception of a signal via multiple propagation paths also results in delay spread, which introduces inter-symbol interference within the received signal and may add destructively.
In addition, due to the movement of the mobile unit, the characteristics of the delay spread and its resultant inter-symbol interference can change rapidly with time. Therefore, to optimize reception, propagation signal impairments must be eliminated, or else utilized in ways which can improve the demodulation of the signal data.
To demodulate an impaired signal in wireless technology, the receiver must first estimate the channel via a training sequence, which is a sequence of symbols either prefixed, postfixed, or positioned in the middle of the frame to a transmitted sequence of symbols, and is known by the receiver. Assuming that the model for the impairments, in the form of multi-tap channel coefficients, is known by the receiver, and that the input-output characteristics of the channel may be regarded as constant during at least one symbol interval, the MLSE (Maximum likelihood Sequence Estimation) equalizer process can be used to demodulate the received signal. This detection technique uses an estimated model for the channel characteristics to update the branch metrics used in a Viterbi decoding process. Since the channel characteristics are a function of the movement of the mobile station, the channel may vary every signaling interval. Therefore, for reliable demodulation performance under severe channel impairment conditions, frequent updates of the channel model are required.
In prior art approaches to the demodulation of a signal with a delay spread impairment, a decision feedback equalizer is often used. Also, the General MLSE approach and adaptive MLSE approaches are used as well. However, most commonly used prior art channel estimation techniques may not provide sufficient accuracy, or they may suffer from excessive lag, due to inherent decision delays. As such, overall receiver performance can be significantly degraded.
Accordingly, it is an object of the present invention to overcome the disadvantages of the prior art by providing an adaptive-predictive estimation scheme which provides a reliable demodulation process for a received signal with severe channel impairments.